Industrial turbines, particularly steam turbines, convert the energy of a hot gas, such as steam, to rotary mechanical energy by employing turbine buckets disposed around the periphery of a rotor in a series of turbine stages as is well known. In order to improve the conversion efficiency of the turbine, it is desirable to increase the length of the buckets in the various stages of the turbine. In order to increase the length of the buckets, particularly at the last or “L-0” stage of the turbine, it has generally been necessary to employ lightweight materials, such as titanium, in order to avoid having to make significant changes in the rotor alloys, which are typically NiCrMoV steel alloys, because the use of heavier bucket materials, such as high strength steels, in long bucket configurations generally create stresses that exceed the operating limits of these rotor alloys. While the use of higher strength materials in the turbine rotors is possible, it has generally been undesirable because the higher strength materials typically have a significantly higher cost both for the alloy materials themselves and the processes needed to fabricate rotors from these materials.
Therefore, it is desirable to develop turbines that are configured to employ high strength steel buckets in the last turbine stage without necessitating a change in the NiCrMoV steel alloys typically used to form the turbine rotors.